Optical fiber structures and methods for varying laser beam profile

ABSTRACT

In various embodiments, the beam parameter product and/or numerical aperture of a laser beam is adjusted utilizing a step-clad optical fiber having a central core, a first cladding, an annular core, and a second cladding.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/318,959, filed Apr. 6, 2016, the entiredisclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to laser systems,specifically laser systems with controllable beam profiles, e.g.,variable beam parameter products.

BACKGROUND

High-power laser systems are utilized for a host of differentapplications, such as welding, cutting, drilling, and materialsprocessing. Such laser systems typically include a laser emitter, thelaser light from which is coupled into an optical fiber (or simply a“fiber”), and an optical system that focuses the laser light from thefiber onto the workpiece to be processed. The optical system istypically engineered to produce the highest-quality laser beam, or,equivalently, the beam with the lowest beam parameter product (BPP). TheBPP is the product of the laser beam's divergence angle (half-angle) andthe radius of the beam at its narrowest point (i.e., the beam waist, theminimum spot size). That is, BPP=NA×D/2, where D is the focusing spot(the waist) diameter and NA is the numerical aperture; thus, the BPP maybe varied by varying NA and/or D. The BPP quantifies the quality of thelaser beam and how well it can be focused to a small spot, and istypically expressed in units of millimeter-milliradians (mm-mrad). AGaussian beam has the lowest possible BPP, given by the wavelength ofthe laser light divided by pi. The ratio of the BPP of an actual beam tothat of an ideal Gaussian beam at the same wavelength is denoted M²,which is a wavelength-independent measure of beam quality.

In many laser-processing applications, the desired beam spot size,divergence, and beam quality may vary depending on, for example, thetype of processing and/or the type of material being processed. This isparticularly true for industrial lasers in material processingapplications. For example, a lower BPP value, i.e., a better beamquality, may be preferred for cutting a thin metal, while a larger BPP(i.e., a worse beam quality) may be preferred for cutting throughthicker metals. In order to make such changes to the BPP of the lasersystem, frequently the output optical system or the optical fiber mustbe swapped out with other components and/or realigned, a time-consumingand expensive process that may even lead to inadvertent damage of thefragile optical components of the laser system. Thus, there is a needfor alternative techniques for varying the BPP of a laser system that donot involve such adjustments to the laser beam or optical system at theoutput of the optical fiber.

SUMMARY

Various embodiments of the present invention provide laser systems inwhich the BPP of the system (i.e., of its output laser beam) is variedvia manipulation of the numerical aperture (NA) and the spot size (D) ofthe laser system while minimizing or substantially eliminating opticalpower losses. Embodiments of the invention involve coupling of the laserbeam into a multi-clad optical fiber herein termed a “step-clad fiber.”One exemplary step-clad fiber includes, consists essentially of, orconsists of a center core, a first cladding disposed around the centercore, the first annular core disposed around the first cladding, and asecond cladding disposed around the first annular core. Step-clad fibersin accordance with embodiments of the invention are not limited tohaving only a single annular core and two claddings—one or moreadditional annular cores and associated claddings may be disposed aroundthe second cladding. As utilized herein, the term “annular core” isdefined as a ring-shaped region having a higher refractive index thanboth the inner and outer layers adjacent thereto. Layers other than thecenter core and the annular core(s) are typically claddings in astep-clad fiber. Such claddings have lower refractive indices than atleast one layer adjacent thereto.

When conventional optical fibers are utilized in laser systems, laserpower is typically intended to be only coupled into the cores. Any power“over-spray” on the claddings results in a loss of power and/or becomesa harmful factor to the optics downstream to the fiber. In contrast,laser systems utilizing step-clad fibers in accordance with embodimentsof the present invention intentionally have partial or full laser powercoupled into at least the first cladding, which is specifically designedwith a higher refractive index and larger diameter than those of aninternal cladding layer of a conventional multi-clad fiber. For example,the diameter of a first cladding in accordance with embodiments of theinvention may range from about 140 μm to about 180 μm for a fiber havinga central core diameter of 100 μm, compared to only 110 μm to 120 μm forconventional standard or multi-clad 100-μm-core optical fiber. Invarious embodiments, the thickness of the first cladding layer may rangefrom, e.g., approximately 40 μm to approximately 100 μm, orapproximately 40 to approximately 80 μm.

In embodiments of the invention, utilization of the step-clad fiberenables the variation of laser-system BPP based on not only thediameters of the cores and the power ratios in the cores, but also thethickness and refractive index of the first cladding and the powerratios in the first cladding (and/or in additional claddings, ifpresent). The laser power coupled into the first cladding of thestep-clad fiber is confined by the first annular core and enables a highdegree of BPP variation. Moreover, coupling into the annular core(s) inaccordance with embodiments of the invention enables the formation ofoutput beams having uniform profiles without “gaps” or areas of low orno laser power therein; in contrast, conventional techniques in whichcoupling into cladding layers is impossible and/or actively avoidedgenerally result in output beams having gaps (e.g., annular gaps) in theoutput beam profile. While embodiments of the invention may be utilizedto produce output beam profiles that vary as a function of radius ordiameter, even areas between high-intensity areas typically containlaser beam intensity (and are thus not “empty”), and/or anylow-intensity or empty regions are very limited in spatial extent,particularly in comparison to output beams produced by conventionaltechniques.

Embodiments of the invention even include variations in whichsubstantially no power is coupled into the one or more annular coreregions of the step-clad fiber. Embodiments of the present inventiontypically vary BPP of the laser system via variation of both the outputspot size D and NA and therefore provide a larger range of BPP variationthan conventional techniques. For a given BPP variation range,embodiments of the present invention enable use of a step-clad fiberhaving a much smaller diameter of the first annular core than would beneeded in conventional systems; thus, embodiments of the inventiongenerate high-BPP laser beams of much smaller spot size with much lessdiluted power density. That is, a smaller change in the spot size D isneeded in accordance with embodiments of the invention to vary the BPP(since the NA may also be simultaneously varied), thereby leading toless dilution of power density. For example, the diameter of a firstannular core in accordance with embodiments of the invention may rangefrom approximately 300 μm to approximately 400 μm (e.g., approximately360 μm). The thickness of a first annular core in accordance withvarious embodiments may be, for example, approximately 60 μm toapproximately 150 μm, approximately 80 μm to approximately 120 μm,approximately 90 μm to approximately 110 μm, or approximately 100 μm.

Herein, “optical elements” may refer to any of lenses, mirrors, prisms,gratings, and the like, which redirect, reflect, bend, or in any othermanner optically manipulate electromagnetic radiation, unless otherwiseindicated. Herein, beam emitters, emitters, or laser emitters, or lasersinclude any electromagnetic beam-generating device such as semiconductorelements, which generate an electromagnetic beam, but may or may not beself-resonating. These also include fiber lasers, disk lasers, non-solidstate lasers, etc. Generally, each emitter includes a back reflectivesurface, at least one optical gain medium, and a front reflectivesurface. The optical gain medium increases the gain of electromagneticradiation that is not limited to any particular portion of theelectromagnetic spectrum, but that may be visible, infrared, and/orultraviolet light. An emitter may include or consist essentially ofmultiple beam emitters such as a diode bar configured to emit multiplebeams. The input beams received in the embodiments herein may besingle-wavelength or multi-wavelength beams combined using varioustechniques known in the art.

Embodiments of the invention may be utilized with wavelength beamcombining (WBC) systems that include a plurality of emitters, such asone or more diode bars, that are combined using a dispersive element toform a multi-wavelength beam. Each emitter in the WBC systemindividually resonates, and is stabilized through wavelength-specificfeedback from a common partially reflecting output coupler that isfiltered by the dispersive element along a beam-combining dimension.Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed onFeb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat.No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107,filed on Mar. 7, 2011, the entire disclosure of each of which isincorporated by reference herein. Multi-wavelength output beams of WBCsystems may be utilized as input beams in conjunction with embodimentsof the present invention for, e.g., BPP control.

In an aspect, embodiments of the invention feature a laser system thatincludes, consists essentially of, or consists of a beam source foremission of an input laser beam, a step-clad optical fiber having aninput end and an output end opposite the input end, an in-couplingmechanism, and a controller. The step-clad optical fiber includes,consists essentially of, or consists of (i) a central core having afirst refractive index, (ii) surrounding the central core, a firstcladding having a second refractive index, (iii) surrounding the firstcladding, an annular core having a third refractive index, and (iv)surrounding the annular core, a second cladding having a fourthrefractive index. The first refractive index is larger than the fourthrefractive index. The third refractive index is larger than the fourthrefractive index. The second refractive index is smaller than the firstrefractive index and larger than the fourth refractive index. Thein-coupling mechanism receives the input laser beam and directs theinput laser beam toward the input end of the step-clad optical fiber,whereby the input laser beam is in-coupled into the step-clad opticalfiber and emitted from the output end of the step-clad optical fiber asan output beam. The controller controls the in-coupling mechanism todirect the input laser beam onto one or more in-coupling locations onthe input end of the step-clad optical fiber, whereby at least one of abeam parameter product or a numerical aperture of the output beam isdetermined at least in part by the one or more in-coupling locations.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least one of the in-couplinglocations may at least partially overlap the central core. At least oneof the in-coupling locations may at least partially overlap the firstcladding. At least one of the in-coupling locations may at leastpartially overlap the annular core. At least one of the in-couplinglocations may at least partially overlap the second cladding. At leastone of the in-coupling locations may at least partially overlap two ormore of the central core, the first cladding, the annular core, or thesecond cladding. The in-coupling mechanism may include, consistessentially of, or consist of an optical element for focusing the inputlaser beam toward the input end of the step-clad optical fiber. Theoptical element may be movable (e.g., translatable, rotatable, and/ortiltable), in response to the controller, along (i) an axissubstantially parallel to a propagation direction of the input laserbeam and/or (ii) one or more axes substantially perpendicular to thepropagation direction of the input laser beam. The in-coupling mechanismmay include, consist essentially of, or consist of a reflector forreceiving the input laser beam and reflecting the input laser beamtoward the step-clad optical fiber. The optical element may include,consist essentially of, or consist of one or more lenses, one or moremirrors, and/or one or more prisms. The in-coupling mechanism mayinclude, consist essentially of, or consist of a reflector for receivingthe input laser beam and reflecting the input laser beam toward thestep-clad optical fiber, the reflector being rotatable in response tothe controller. The in-coupling mechanism may include an optical elementfor receiving the input laser beam from the reflector and focusing theinput laser beam toward the step-clad optical fiber. The optical elementmay be movable (e.g., translatable, rotatable, and/or tiltable), inresponse to the controller, along (i) an axis substantially parallel toa propagation direction of the input laser beam and/or (ii) one or moreaxes substantially perpendicular to the propagation direction of theinput laser beam. The optical element may include, consist essentiallyof, or consist of one or more lenses, one or more mirrors, and/or one ormore prisms.

The beam source may be responsive to the controller. The controller maybe configured to direct the input laser beam onto a plurality ofdifferent in-coupling locations without modulating an output power of(e.g., modulating the intensity of or switching off) the input laserbeam as the input laser beam is directed between the differentin-coupling locations. The controller may be configured to direct theinput laser beam onto at least one in-coupling location at leastpartially overlapping the first cladding, whereby beam energy in-coupledinto the first cladding forms at least a portion of the output beam.Beam energy in-coupled into the first cladding may be confined withinthe step-clad optical fiber at the interface between the annular coreand the second cladding. The second refractive index may be smaller thanthe third refractive index. The second refractive index may beapproximately equal to the third refractive index. The third refractiveindex may be smaller than the first refractive index. The thirdrefractive index may be greater than the first refractive index. Thebeam source may include, consist essentially of, or consist of one ormore beam emitters (each and/or collectively) emitting a plurality ofdiscrete beams, focusing optics, a dispersive element, and a partiallyreflective output coupler. Each of the discrete beams may have adifferent wavelength. The focusing optics may focus the plurality ofbeams onto the dispersive element. The dispersive element may receiveand disperse (i.e., wavelength disperses) the received focused beams.The partially reflective output coupler may be positioned to receive thedispersed beams, transmit a portion of the dispersed beams therethroughas the input laser beam, and reflect a second portion of the dispersedbeams back toward the dispersive element. The input laser beam may becomposed of multiple wavelengths. The dispersive element may include oneor more diffraction gratings (e.g., a transmissive grating and/or areflective grating).

In another aspect, embodiments of the invention feature a method ofadjusting a beam parameter product and/or a numerical aperture of alaser beam. A step-clad optical fiber having an input end and an outputend opposite the input end is provided. The step-clad optical fiberincludes, consists essentially of, or consists of (i) a central corehaving a first refractive index, (ii) surrounding the central core, afirst cladding having a second refractive index, (iii) surrounding thefirst cladding, an annular core having a third refractive index, and(iv) surrounding the annular core, a second cladding having a fourthrefractive index. The first refractive index is larger than the fourthrefractive index. The third refractive index is larger than the fourthrefractive index. The second refractive index is smaller than the firstrefractive index and larger than the fourth refractive index. An inputlaser beam is directed onto one or more in-coupling locations on theinput end of the step-clad optical fiber, whereby (i) the input laserbeam is in-coupled into the step-clad optical fiber and emitted from theoutput end of the step-clad optical fiber as an output beam, and (ii) atleast one of a beam parameter product of a numerical aperture of theoutput beam is determined at least in part by the one or morein-coupling locations.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least one of the in-couplinglocations may at least partially overlap the central core. At least oneof the in-coupling locations may at least partially overlap the firstcladding. At least one of the in-coupling locations may at leastpartially overlap the annular core. At least one of the in-couplinglocations may at least partially overlap the second cladding. At leastone of the in-coupling locations may at least partially overlap two ormore of the central core, the first cladding, the annular core, or thesecond cladding. The input laser beam may be directed onto a pluralityof different in-coupling locations to produce a single output beam ormultiple output beams having different numerical apertures and/or beamparameter products. The output power of the input laser beam may not bemodulated (e.g., decreased or switched off) as the input laser beam isdirected between the different in-coupling locations. At least one ofthe in-coupling locations may at least partially overlap the firstcladding, whereby beam energy in-coupled into the first cladding formsat least a portion of the output beam. Beam energy in-coupled into thefirst cladding may be confined within the step-clad optical fiber at theinterface between the annular core and the second cladding. The secondrefractive index may be smaller than the third refractive index. Thesecond refractive index may be approximately equal to the thirdrefractive index. The third refractive index may be smaller than thefirst refractive index. The third refractive index may be greater thanthe first refractive index. The input laser beam may be emitted by abeam source. The beam source may include, consist essentially of, orconsist of one or more beam emitters (each and/or collectively) emittinga plurality of discrete beams, focusing optics, a dispersive element,and a partially reflective output coupler. Each of the discrete beamsmay have a different wavelength. The focusing optics may focus theplurality of beams onto the dispersive element. The dispersive elementmay receive and disperse (i.e., wavelength disperses) the receivedfocused beams. The partially reflective output coupler may be positionedto receive the dispersed beams, transmit a portion of the dispersedbeams therethrough as the input laser beam, and reflect a second portionof the dispersed beams back toward the dispersive element. The inputlaser beam may be composed of multiple wavelengths. The dispersiveelement may include one or more diffraction gratings (e.g., atransmissive grating and/or a reflective grating).

In yet another aspect, embodiments of the invention feature a method ofadjusting at least one of a beam parameter product or a numericalaperture of a laser beam. A step-clad optical fiber having an input endand an output end opposite the input end is provided. The step-cladoptical fiber includes, consists essentially of, or consists of (i) acentral core having a first refractive index, (ii) surrounding thecentral core, a first cladding having a second refractive index, (iii)surrounding the first cladding, an annular core having a thirdrefractive index, and (iv) surrounding the annular core, a secondcladding having a fourth refractive index. The third refractive index islarger than the fourth refractive index. The third refractive index islarger than the second refractive index. An input laser beam is directedonto one or more in-coupling locations on the input end of the step-cladoptical fiber, whereby (i) the input laser beam is in-coupled into thestep-clad optical fiber and emitted from the output end of the step-cladoptical fiber as an output beam, and (ii) at least one of a beamparameter product or a numerical aperture of the output beam isdetermined at least in part by the one or more in-coupling locations. Atleast one of the in-coupling locations at least partially overlaps thefirst cladding, whereby beam energy in-coupled into the first claddingforms at least a portion of the output beam.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least one of the in-couplinglocations may at least partially overlap the central core. At least oneof the in-coupling locations may at least partially overlap the annularcore. At least one of the in-coupling locations may at least partiallyoverlap the second cladding. At least one of the in-coupling locationsmay at least partially overlap two or more of the central core, thefirst cladding, the annular core, or the second cladding. The inputlaser beam may be directed onto a plurality of different in-couplinglocations to produce a single output beam or multiple output beamshaving different numerical apertures and/or beam parameter products. Theoutput power of the input laser beam may not be modulated (e.g.,decreased or switched off) as the input laser beam is directed betweenthe different in-coupling locations. The input laser beam may be emittedby a beam source. The beam source may include, consist essentially of,or consist of one or more beam emitters (each and/or collectively)emitting a plurality of discrete beams, focusing optics, a dispersiveelement, and a partially reflective output coupler. Each of the discretebeams may have a different wavelength. The focusing optics may focus theplurality of beams onto the dispersive element. The dispersive elementmay receive and disperse (i.e., wavelength disperses) the receivedfocused beams. The partially reflective output coupler may be positionedto receive the dispersed beams, transmit a portion of the dispersedbeams therethrough as the input laser beam, and reflect a second portionof the dispersed beams back toward the dispersive element. The inputlaser beam may be composed of multiple wavelengths. The dispersiveelement may include one or more diffraction gratings (e.g., atransmissive grating and/or a reflective grating).

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, the term“substantially” means ±10%, and in some embodiments, ±5%. The term“consists essentially of” means excluding other materials thatcontribute to function, unless otherwise defined herein. Nonetheless,such other materials may be present, collectively or individually, intrace amounts. Herein, the terms “radiation” and “light” are utilizedinterchangeably unless otherwise indicated. Herein, “downstream” or“optically downstream,” is utilized to indicate the relative placementof a second element that a light beam strikes after encountering a firstelement, the first element being “upstream,” or “optically upstream” ofthe second element. Herein, “optical distance” between two components isthe distance between two components that is actually traveled by lightbeams; the optical distance may be, but is not necessarily, equal to thephysical distance between two components due to, e.g., reflections frommirrors or other changes in propagation direction experienced by thelight traveling from one of the components to the other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a schematic diagram of a conventional double-clad fiber andoptical rays transmitted into the core and a cladding thereof;

FIG. 1B is a schematic diagram of refractive indices of the variouslayers of the fiber of FIG. 1A;

FIG. 1C is a plan view of a typical laser beam emanating from the fiberof FIG. 1A;

FIG. 2A is a schematic diagram of a step-clad fiber in accordance withvarious embodiments of the invention and optical rays transmitted intothe center core and first cladding thereof;

FIG. 2B is a schematic diagram of refractive indices of the variouslayers of the step-clad fiber of FIG. 2A in accordance with embodimentsof the invention;

FIG. 2C is a plan view of a laser beam emanating from the step-cladfiber of FIG. 2A in accordance with embodiments of the invention;

FIG. 3A is a schematic diagram of a step-clad fiber in accordance withvarious embodiments of the invention;

FIG. 3B is a schematic diagram of refractive indices of the variouslayers of the step-clad fiber of FIG. 3A in accordance with embodimentsof the invention;

FIG. 4A is a schematic diagram of portions of a laser system utilizing astep-clad fiber in accordance with embodiments of the invention;

FIG. 4B is a graph of variation in output BPP as a function of reflectortilt for the laser system of FIG. 4A in accordance with embodiments ofthe invention;

FIG. 4C is a graph of variation of output NA as a function of reflectortilt for the laser system of FIG. 4A in accordance with embodiments ofthe invention;

FIGS. 5 and 6 are schematic diagrams of portions of laser systemsutilizing a step-clad fiber and adjustable lenses in accordance withembodiments of the invention; and

FIG. 7 is a schematic diagram of a wavelength beam combining lasersystem that may be utilized to supply the input beam for laser systemsin accordance with various embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1A shows a conventional double-clad fiber 100 having a center core105, an interior cladding 110, an annular core 115, and an exteriorcladding 120. The radius of each layer (core or cladding) of the fiber100 is represented by R₁, R₂, R₃, or R₄, as shown in FIG. 1B. Inconventional double-clad fiber 100, the two cores 105, 115 typicallyhave the same higher refractive index, N_(H), and the two claddings 110,120 typically have the same lower refractive index, N_(L), as shown inFIG. 1B, and, therefore, the two cores 105, 115 have the same NA ofsqrt(N_(H) ²−N_(L) ²).

FIG. 1A also depicts the transmission of three representative light raysin the fiber 100. Ray 125 is coupled into the center core 105, isconfined by the center core 105, and exits from the center core 105 atthe same angle as that which it entered the center core 105. Ray 130 andray 135 are transmitted into the interior cladding 110, propagate withinthe entire fiber area, and are confined by the exterior cladding 120.Since the exit surface (the right edge of the fiber 100 in FIG. 1A)contains regions having two different refractive indices, ray 130coupled into the interior cladding (which has a low refractive index)and exiting from the low refractive index region at the exit surfacewill typically have an exit angle equal to the input angle. Ray 135coupled into the interior cladding and exiting from the higherrefractive index region at the exit surface will typically have an exitangle larger than the input angle, as shown in FIG. 1A.

FIG. 1C is an image of an output profile 140 observed at a distance fromthe exit surface of conventional double-clad fiber 100 when a laser beamis transmitted into both the center core 105 and the interior cladding110. As shown, output profile 140 contains two well-separated areas ofbeam intensity, a round central area 145 and an outer-ring area 150.Areas 145, 150 correspond to collections of two groups of rays belongingto two clusters of exiting angles from the fiber 100, as detailed above.The rays similar to ray 125 and ray 130 will exit at angles equal totheir input angles and will contribute to the central round area 145,while the rays similar to ray 135 will exit at angles larger than theirinput angles and, therefore, form the outer-ring area 150. To a firstapproximation, if not considering any NA degradations due to fiberbending and non-uniformity, the NA of the central round area beam istypically the same as the laser input NA. However, the NA of theouter-ring beam 150, calculated by sqrt(NA_(F) ²+NA_(IN) ²), issubstantially larger than the input NA (NA_(IN)) and also larger thanthe fiber NA (NA_(F)), where NA_(F)=sqrt(N_(H) ²−N_(L) ²).

The interior cladding 100 of double-clad fiber 100 is relatively thinand is not intended for the in-coupling of laser energy. For 100 μm-coreconventional double-clad fiber 100, the diameter of the first claddingwill be normally be in the range of approximately 110-120 μm, i.e., alayer thickness of about 5-10 μm. However, the beam power entering theinterior cladding of the fiber 100, as represented by the rays 130, 135in FIG. 1A, will necessarily spread over to the outer cladding 120 andis typically either removed by mode strippers or transmitted through thefiber 100 with the majority forming the outer-ring area 150 shown inFIG. 1C. The former may post a great risk of burning the mode strippersand the fiber 100, and the latter nay damage the optics downstream ofthe fiber 100 due to the large NA of the outer-ring beam 150.

Embodiments of the present invention include laser systems utilizingstep-clad optical fibers, as illustrated in FIG. 2A. In accordance withvarious embodiments, a step-clad fiber 200 includes, consistsessentially of, or consists of a center core 205, a first cladding 210,an annular core 215, and a second cladding 220. Advantageously, variousproperties of the first cladding 210 enable BPP variation based at leastin part on the power coupled into the first cladding 210. FIG. 2Cdepicts an exemplary output profile 225 for laser beam energy coupledinto the step-clad fiber 200. As shown, output profile 225 includes acentral round area 230 and an outer-ring area 235 if observed at adistance from the fiber 200. However, the outer-ring area 235 shown inFIG. 2C typically has a much smaller NA than the corresponding area 150depicted in FIG. 1C; thus, energy in area 235 may be safely accepted bythe optics downstream to the fiber 200. For example, the outer-ring area235 may have an NA of less than 0.18, less than 0.17, less than 0.16, oreven less than 0.15 for an input laser NA of 0.1. In contrast, thecorresponding area 150 may have an NA of at least 0.24 for an inputlaser NA of 0.1. In various embodiments, the NA of any outer-ring area235 may be less than approximately 180% of the NA of the input laser,less than approximately 170% of the NA of the input laser, less thanapproximately 160% of the NA of the input laser, or even less thanapproximately 150% of the NA of the input laser. In various embodiments,the difference between the NA of any outer-ring area and the NA of theinput laser may be less than approximately 0.08, less than approximately0.07, less than approximately 0.06, or even less than approximately0.05. In various embodiments, the difference between the NA of anyouter-ring area and the NA of the input laser may be at leastapproximately 0.005, or even at least approximately 0.01. In variousembodiments, the difference between the NA of any outer-ring area andthe NA of the step-clad fiber may be less than 0.04, less than 0.03,less than 0.02, or less than 0.01. In various embodiments, thedifference between the NA of any outer-ring area and the NA of thestep-clad fiber may be at least 0.001, or even at least 0.005.

As depicted in FIG. 2A, a ray 240 transmitted into the center core 205will typically be confined by the center core 205. Rays 245, 250transmitted into the first cladding 210 will generally be confined bythe first annular core 215. Ray 240, 245 will typically exit at anglesequal to their corresponding input angles and will form the centralround area 230 shown in FIG. 2C, while ray 250 will exit at a slightlylarger angle than its corresponding input angle and will form theouter-ring area 235. In stark contrast with fiber 100 described above,rays emitted into the first cladding 210 generally will not reach theouter cladding 220 and, therefore, will not pose a risk of damaging(e.g., burning out) the fiber 200 or its associated optics. In variousembodiments of the present invention, the step-clad fiber 200 isconfigured for the in-coupling of all or part of the input laser powerinto the first cladding 210. Such in-coupled power will be not be lostor pose a risk of damaging the fiber 200; rather, it may be a majorcontributor to BPP variation of the output beam.

FIG. 2B depicts the refractive index and radius of each layer of thestep-clad fiber 200. In contrast with the refractive indices of fiber100 shown in FIG. 1B, the refractive index (N_(M)) of the first cladding210 of the fiber 200 has a value between a high index N_(H) (notnecessarily the high index of FIG. 1B) and a low index N_(L) (notnecessarily the low index of FIG. 1B), so that the center core 205 willhave a smaller NA, given by sqrt(N_(H) ²−N_(M) ²), than the NA of theannular core 215, given by sqrt(N_(H) ²−N_(L) ²). While FIG. 2B depictsthe indices of refraction of the center core 205 and the annular core215 as being approximately equal to each other, in various embodimentsthe index of refraction of the annular core 215 may be different from(i.e., either less than or greater than) the index of refraction of thecenter core 205; however, in general, the index of refraction of theannular core 215 remains larger than the index of refraction of thefirst cladding 210.

For a given laser input NA (NA_(IN)), the difference between indices ofrefraction N_(H) and N_(M) will at least partially define the NA of theouter-ring beam 235 shown in FIG. 2C, and this NA is given by sqrt(N_(H)²−N_(M) ²+NA_(IN) ²). The smaller the index difference between N_(H) andN_(M), the smaller will be the outer-ring NA. However, reducing theindex difference of N_(H) and N_(M) will typically also decrease the NAof the center core 205 and, therefore, may result in more rays escapingfrom the center core 205. Although such rays will be confined by theannular core 215, the rays escaping from the center core 205 may degradethe best possible BPP, i.e., may result in a higher initial BPP valuefor the output beam.

In various embodiments, the NA of center core 205 of the step-clad fiber200 ranges from approximately 0.07 to approximately 0.17, or even fromapproximately 0.09 to approximately 0.14. In various embodiments, theeffective NA of the first cladding 210 is larger than approximately0.09, or even larger than approximately 0.12. In various embodiments,the refractive index of the first annular core 215 is equal or smallerthan the refractive index of the center core 205.

In various embodiments, the first annular core has the same refractiveindex as the first cladding, as shown in FIGS. 3A and 3B, in which thefirst annular core from FIG. 2A has merged into the first cladding. Asshown, such a step-clad fiber 300 includes, consists essentially of, orconsists of a center core 305, a first cladding 310, and a secondcladding 315. As shown in FIG. 3B, the refractive index (N_(M)) of thefirst cladding 310 is between the refractive indices of the center core(N_(H)) and the second cladding (N_(L)). (All refractive index valuesare not necessarily the same as values depicted in FIGS. 1B and 2B.) Invarious embodiments, the NA of the center core 305 of step-clad fiber300 ranges from approximately 0.07 to approximately 0.17, or even fromapproximately 0.09 to approximately 0.14. In various embodiments, the NAof the first cladding 310 of the step-clad fiber 300 is larger thanapproximately 0.09, or even larger than approximately 0.12.

As mentioned herein, step-clad fibers in accordance with embodiments ofthe invention may have substantially all or all of the laser powercoupled into the first cladding. More power coupled into the firstcladding will generally lead to larger BPP. In various embodiments, thediameter ratio of the first cladding and the center core is larger than1.2, e.g., between 1.2 and 3, or even between 1.3 and 2.

The maximum BPP obtainable with a step-clad fiber in accordance withembodiments of the invention may be dependent on the diameter of thefirst annular core (or the diameter of the first cladding if the firstannular core is absent). Therefore, in various embodiments, the diameterratio of the first annular core (or the first cladding if the firstannular core is absent) and the center core ranges from approximately1.5 to approximately 6.5, or even from approximately 2 to approximately5.

Structurally, optical fibers in accordance with embodiments of theinvention may include one or more layers of high and/or low refractiveindex beyond (i.e., outside of) the second cladding without altering theprinciples of the present invention. Such additional layers may also betermed claddings and annular cores, but may not guide light. Suchvariants are within the scope of the present invention. In accordancewith various embodiments of the invention, the various core and claddinglayers of step-clad fibers may include, consist essentially of, orconsist of glass, such as substantially pure fused silica and/or fusedsilica doped with fluorine, titanium, germanium, and/or boron.

An exemplary laser system 400 for varying BPP using a step-clad fiber200 in accordance with embodiments of the invention is depicted in FIG.4. (Such systems may alternatively utilize step-clad fiber 300 inaccordance with embodiments of the invention.) As shown, the lasersystem 400 includes an adjustable reflector 405 (e.g., a tip-tiltadjustable mirror) to redirect an incoming input laser beam 410 to afiber coupling optical element 415 (e.g., one or more lenses), whichfocuses the beam 410 toward the step-clad fiber 200. As shown, theregion of the input face of step-clad fiber 200 at which the beam 410 isin-coupled is at least partially defined by the configuration (e.g., theposition and/or angle) of the reflector 405. For the best starting beamquality (i.e., the smallest BPP), the step-clad fiber 200 is typicallylocated at the focal spot of the optical element 415.

The configuration of the reflector 405 may be controlled via acontroller 420 and/or one or more actuators (not shown) operativelyconnected to the reflector 405. Thus, the reflector 405 and/or the oneor more actuators may be responsive to controller 420. The controller420 may be responsive to a desired target radiation power distributionand/or BPP or other measure of beam quality (e.g., input by a userand/or based on one or more properties of a workpiece to be processedsuch as the distance to the workpiece, the composition of the workpiece,topography of the workpiece, etc.) and configured to angle reflector 405to cause the beam 410 to strike the input face of the step-clad fiber200 such that the output beam output from the step-clad fiber 200 hasthe target radiation power distribution or beam quality. The output beamthus produced may be directed to a workpiece for processes such asannealing, cutting, welding, drilling, etc.

The controller 420 may be programmed to achieve the desired powerdistribution and/or output BPP and/or beam quality via a particularreflector tilt as detailed herein.

The controller 420 may be provided as either software, hardware, or somecombination thereof. For example, the system may be implemented on oneor more conventional server-class computers, such as a PC having a CPUboard containing one or more processors such as the Pentium or Celeronfamily of processors manufactured by Intel Corporation of Santa Clara,Calif., the 680x0 and POWER PC family of processors manufactured byMotorola Corporation of Schaumburg, Ill., and/or the ATHLON line ofprocessors manufactured by Advanced Micro Devices, Inc., of Sunnyvale,Calif. The processor may also include a main memory unit for storingprograms and/or data relating to the methods described herein. Thememory may include random access memory (RAM), read only memory (ROM),and/or FLASH memory residing on commonly available hardware such as oneor more application specific integrated circuits (ASIC), fieldprogrammable gate arrays (FPGA), electrically erasable programmableread-only memories (EEPROM), programmable read-only memories (PROM),programmable logic devices (PLD), or read-only memory devices (ROM). Insome embodiments, the programs may be provided using external RAM and/orROM such as optical disks, magnetic disks, as well as other commonlyused storage devices. For embodiments in which the functions areprovided as one or more software programs, the programs may be writtenin any of a number of high level languages such as FORTRAN, PASCAL,JAVA, C, C++, C#, BASIC, various scripting languages, and/or HTML.Additionally, the software may be implemented in an assembly languagedirected to the microprocessor resident on a target computer; forexample, the software may be implemented in Intel 80x86 assemblylanguage if it is configured to run on an IBM PC or PC clone. Thesoftware may be embodied on an article of manufacture including, but notlimited to, a floppy disk, a jump drive, a hard disk, an optical disk, amagnetic tape, a PROM, an EPROM, EEPROM, field-programmable gate array,or CD-ROM.

Simulation results of the output BPP and NA of the laser system 400 aredepicted in FIGS. 4B and 4C, respectively. In FIGS. 4B and 4C, theoptical element 415 has a 30 mm focal length, and the step-clad fiber200 has diameters of 100 μm, 160 μm, 360 μm, and 400 μm for the centercore, the first cladding, the first annular core, and the secondcladding, respectively. The NAs of the center core and the first annularcore are 0.12 and 0.22, respectively. As shown in FIG. 4B, the BPPranges from approximately 4 mm-mrad and reaches a maximum atapproximately 20 mm-mrad at reflector tilt of about 1.2 mrad. The BPPvariation is a combined result of output spot size (D) change and NAchange. As the reflector tilt gradually increases from its initial zeroposition, more power is coupled into the first cladding, whicheffectively enlarges the output spot size (D) and the NA and thus theBPP. Further increases of reflector tilt (above approximately 1.2 mrad)result, in this example, in smaller NA due to decreased power coupledinto the first cladding. FIG. 4C indicates that the output NA decreasesback to its initial smallest NA (obtained at 0 reflector tilt) when thereflector tilt is larger than approximately 2.2 mrad, at which point allthe power is coupled into the first annular core. As evident from FIGS.4B and 4C, the power coupled into the first cladding is not lost, butinstead plays a major role in BPP variation.

In addition, the maximum BPP of approximately 20 mm-mrad for thisexemplary embodiment is obtained with a step-clad fiber in which thediameter of the first annular core is 360 μm. For the same BPP range,conventional techniques would require a double-clad fiber having a firstannular core diameter of 500 μm, which would result in an almosttwo-times lower power density at the laser focal spot than in theexemplary embodiment of the present invention. Thus, as indicated,embodiments of the invention advantageously generate larger BPP withincreased NA, which is generally desired for large-BPP applications (to,e.g., generate and maintain higher power density of the output beam).

FIG. 5 depicts a laser system 500 in accordance with embodiments of theinvention with a step-clad fiber (51). In laser system 500, thecontroller 420 directly controls the optical element 415, which isadjustable (i.e., translatable) in the directions (i.e., x and y asshown in FIG. 5) orthogonal to the input propagation direction of inputbeam 410 (i.e., z as shown in FIG. 5). For example, the controller 420may control a lens manipulation system (e.g., one or more motorizedstages or actuators movable in two or three dimensions) to control themovements of optical element 415 and thereby adjust the in-couplingposition of the input beam 410 on the input surface of step-clad fiber200 (and, thus, the BPP of the output beam emerging from the step-cladfiber 200). As with laser system 400, the relative amounts of the inputbeam 400 coupled into the various regions of step-clad fiber 200 resultsin a controllable variable BPP and/or NA of the output beam. FIG. 6depicts a similar laser system 600, in which the optical element 415 isadjustable in the direction parallel to the input propagation directionof input beam 410 (i.e., z as shown in FIG. 6) via, e.g., one or moremotorized stages or actuators. Translation of the optical element 415with respect to the step-clad fiber 200 alters the amount of the beamthat propagates to, and is in-coupled into, the various regions ofstep-clad fiber 200. As with laser system 400, laser systems 500, 600may be utilized with step-clad fiber 300 in addition to or instead ofstep-clad fiber 200. Laser systems in accordance with embodiments of theinvention may also combine translation of optical element 415 (as inFIGS. 5 and 6) with adjustment of reflector 405 (as in FIG. 4) tocontrollably adjust the BPP and/or NA of the beam output from thestep-clad fiber.

Laser systems 400, 500, 600 may be utilized to alter the BPP and/or NAof a laser beam in a continuous fashion without the need to power down(i.e., switch off) the input laser beam as the beam is swept across theinput face of the step-clad fiber such that different portions of thebeam are in-coupled into different regions of the fiber. Because thestep-clad fibers 200, 300 are configured such that beam energypropagating to a cladding region (e.g., the first cladding) is confinedand will not lead to damage to the fiber or optics (e.g., opticalelements) associated therewith, the input beam need not be switched offas it or a portion thereof strikes the cladding(s) of the step-cladfiber.

Embodiments of the present invention may also utilize systems andtechniques of BPP variation as described in U.S. patent application Ser.No. 14/632,283, filed on Feb. 26, 2015, U.S. patent application Ser. No.14/747,073, filed on Jun. 23, 2015, U.S. patent application Ser. No.14/852,939, filed on Sep. 14, 2015, and U.S. patent application Ser. No.15/188,076, filed on Jun. 21, 2016, the entire disclosure of each ofwhich is incorporated by reference herein.

Laser systems and laser delivery systems in accordance with embodimentsof the present invention and detailed herein may be utilized in and/orwith WBC laser systems. Specifically, in various embodiments of theinvention, multi-wavelength output beams of WBC laser systems may beutilized as the input beams for laser beam delivery systems forvariation of BPP as detailed herein. FIG. 7 depicts an exemplary WBClaser system 700 that utilizes one or more lasers 705. In the example ofFIG. 7, laser 705 includes a diode bar having four beam emittersemitting beams 710 (see magnified input view 715), but embodiments ofthe invention may utilize diode bars emitting any number of individualbeams or two-dimensional arrays or stacks of diodes or diode bars. Inview 715, each beam 710 is indicated by a line, where the length orlonger dimension of the line represents the slow diverging dimension ofthe beam, and the height or shorter dimension represents the fastdiverging dimension. A collimation optic 720 may be used to collimateeach beam 710 along the fast dimension. Transform optic(s) 725, whichmay include or consist essentially of one or more cylindrical orspherical lenses and/or mirrors, are used to combine each beam 710 alonga WBC direction 730. The transform optics 725 then overlap the combinedbeam onto a dispersive element 735 (which may include, consistessentially of, or consist of, e.g., a reflective or transmissivediffraction grating, a dispersive prism, a grism (prism/grating), atransmission grating, or an Echelle grating), and the combined beam isthen transmitted as single output profile onto an output coupler 740.The output coupler 740 then transmits the combined beams 745 as shown onthe output front view 750. The output coupler 740 is typically partiallyreflective and acts as a common front facet for all the laser elementsin this external cavity system 700. An external cavity is a lasingsystem where the secondary mirror is displaced at a distance away fromthe emission aperture or facet of each laser emitter. In someembodiments, additional optics are placed between the emission apertureor facet and the output coupler or partially reflective surface. Theoutput beam 745 is a thus a multiple-wavelength beam (combining thewavelengths of the individual beams 710), and may be utilized as theinput beam in laser systems detailed herein and/or may be coupled into astep-clad optical fiber as detailed herein.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is: 1.-31. (canceled)
 32. A step-clad optical fiber foruse in a laser system comprising (i) a beam source for emission of aninput laser beam and (ii) an in-coupling mechanism for receiving theinput laser beam and directing the input laser beam toward an input endof the step-clad optical fiber for emission as an output beam from anoutput end of the step-clad optical fiber opposite the input end, thestep-clad optical fiber consisting of: a central core having a firstrefractive index; surrounding the central core, a first cladding havinga second refractive index; surrounding the first cladding, an annularcore having a third refractive index; and surrounding the annular core,a second cladding having a fourth refractive index, wherein (i) thefirst refractive index is larger than the fourth refractive index, (ii)the third refractive index is larger than the fourth refractive index,and (iii) the second refractive index is smaller than the firstrefractive index and larger than the fourth refractive index.
 33. Thestep-clad optical fiber of claim 32, wherein the second refractive indexis smaller than the third refractive index.
 34. The step-clad opticalfiber of claim 32, wherein the second refractive index is approximatelyequal to the third refractive index.
 35. The step-clad optical fiber ofclaim 32, wherein the third refractive index is smaller than the firstrefractive index.
 36. The step-clad optical fiber of claim 32, whereinthe third refractive index is larger than the first refractive index.37. The step-clad optical fiber of claim 32, wherein a numericalaperture of the first cladding is larger than 0.12.
 38. The step-cladoptical fiber of claim 32, wherein a thickness of the first claddingranges from approximately 40 μm to approximately 100 μm.
 39. Thestep-clad optical fiber of claim 32, wherein a thickness of the annularcore ranges from approximately 60 μm to approximately 150 μm.
 40. Thestep-clad optical fiber of claim 32, wherein a ratio of a diameter ofthe first cladding to a diameter of the central core ranges between 1.2and
 3. 41. The step-clad optical fiber of claim 32, wherein a ratio of adiameter of the annular core to a diameter of the central core rangesbetween 1.5 and 6.5.
 42. A step-clad optical fiber for use in a lasersystem comprising (i) a beam source for emission of an input laser beamand (ii) an in-coupling mechanism for receiving the input laser beam anddirecting the input laser beam toward an input end of the step-cladoptical fiber for emission as an output beam from an output end of thestep-clad optical fiber opposite the input end, the step-clad opticalfiber comprising: a central core having a first refractive index;surrounding the central core, a first cladding having a secondrefractive index; surrounding the first cladding, an annular core havinga third refractive index; and surrounding the annular core, a secondcladding having a fourth refractive index, wherein (i) the firstrefractive index is larger than the fourth refractive index, (ii) thethird refractive index is larger than the fourth refractive index, (iii)the second refractive index is smaller than the first refractive indexand larger than the fourth refractive index, and (iv) the secondrefractive index is approximately equal to the third refractive index.43. The step-clad optical fiber of claim 42, wherein a thickness of thefirst cladding ranges from approximately 40 μm to approximately 100 μm.44. The step-clad optical fiber of claim 42, wherein a thickness of theannular core ranges from approximately 60 μm to approximately 150 μm.45. The step-clad optical fiber of claim 42, wherein a ratio of adiameter of the first cladding to a diameter of the central core rangesbetween 1.2 and
 3. 46. The step-clad optical fiber of claim 42, whereina ratio of a diameter of the annular core to a diameter of the centralcore ranges between 1.5 and 6.5.
 47. A step-clad optical fiber for usein a laser system comprising (i) a beam source for emission of an inputlaser beam and (ii) an in-coupling mechanism for receiving the inputlaser beam and directing the input laser beam toward an input end of thestep-clad optical fiber for emission as an output beam from an outputend of the step-clad optical fiber opposite the input end, the step-cladoptical fiber comprising: a central core having a first refractiveindex; surrounding the central core, a first cladding having a secondrefractive index; surrounding the first cladding, an annular core havinga third refractive index; and surrounding the annular core, a secondcladding having a fourth refractive index, wherein (i) the firstrefractive index is larger than the fourth refractive index, (ii) thethird refractive index is larger than the fourth refractive index, (iii)the second refractive index is smaller than the first refractive indexand larger than the fourth refractive index, and (iv) the thirdrefractive index is larger than the first refractive index.
 48. Thestep-clad optical fiber of claim 47, wherein a thickness of the firstcladding ranges from approximately 40 μm to approximately 100 μm. 49.The step-clad optical fiber of claim 47, wherein a thickness of theannular core ranges from approximately 60 μm to approximately 150 μm.50. The step-clad optical fiber of claim 47, wherein a ratio of adiameter of the first cladding to a diameter of the central core rangesbetween 1.2 and
 3. 51. The step-clad optical fiber of claim 47, whereina ratio of a diameter of the annular core to a diameter of the centralcore ranges between 1.5 and 6.5.